1. Field of the Invention
The present invention relates to a structure of an active matrix type flat panel display integrated with a peripheral drive circuit.
2. Description of the Related Art
An active matrix type liquid crystal display device using an amorphous silicon film has been conventionally known. Also, an active matrix type liquid crystal display device using a crystalline silicon film and being capable of performing higher quality display, has been known.
In the case where the amorphous silicon film is used, there is a problem that a P-channel type thin film transistor can not be realized (its characteristics are too low to be put into practical use). On the other hand, in the case where the crystalline silicon film is used, a practical P-channel type thin film transistor can be manufactured.
Accordingly, in case the crystalline silicon film is used, a CMOS circuit can be constituted by thin film transistors. If using this, a peripheral drive circuit for driving an active matrix circuit can also be constituted by thin film transistors. As shown in FIG. 10, it is possible to realize a structure where an active matrix circuit 10 and peripheral drive circuits 11 and 12 are integrated on a glass substrate or a quartz substrate. Such a structure is called a peripheral drive circuit integration type.
The structure of this peripheral drive circuit integration type has features that the entire of a display device can be made compact, and its manufacturing cost and manufacturing steps can be reduced.
When high picture quality is desired, it is important how fine gradation display can be performed. Generally, a nonsaturated region of a voltage-transmissivity curve of a liquid crystal is used for performing the gradation display. That is, there is adopted a method of performing the gradation display by using the range in which optical response of a liquid crystal is changed in accordance with the change of an applied voltage (electric field). This method is generally called an analog gradation system.
In the case where this analog gradation system is used, the dispersion of voltage applied to the liquid crystal at respective pixels becomes a problem. Especially, when the dispersion of voltage applied to the liquid crystal becomes larger than a voltage required for one gradation, inconsistency occurs in the gradation display. The inconsistency of the gradation display causes display nonuniformity or a striped pattern to be seen.
The dispersion of voltage applied to the liquid crystal at the respective pixels is caused from the dispersion of characteristics of thin film transistors arranged in a matrix form in the unit of several hundred.times.several hundred. In the case of a peripheral drive circuit integration type, the dispersion of characteristics of thin film transistors of the peripheral drive circuit also becomes major cause.
There are generally many parameters relating to the dispersion of characteristics of the thin film transistors. Thus, even if any one parameter is controlled, it is difficult to solve the above problem that the picture quality is degraded. Further, this problem is made more serious since there is such a parameter the dispersion of which can not be completely suppressed.
As described above, since a parameter to be controlled with priority is not clear at present, there is a problem that it is difficult to mass-produce active matrix type display devices having required picture quality while keeping high reproductivity.
In other words, since it is not necessarily clear which parameter should be controlled in what range and in what relation with other parameters, under the present situation, an effort to improve a production yield is made by a short-sighted measure.
Accordingly, the present invention intends to provide guidelines as to which parameter for thin film transistors to be manufactured should be controlled with priority when an active matrix type display device integrated with a peripheral drive circuit is manufactured.
From this, the present invention intends to provide a technique to produce, with a high yield, an active matrix type display device integrated with a peripheral drive circuit which is capable of displaying with a high picture quality.
According to the knowledge of the present inventors, etc., the dispersion of feedthrough voltage at every pixel gives highest influence to the dispersion of liquid crystal driving voltage which highly relates to deterioration of picture quality of a liquid crystal display device.
The influence of a feedthrough voltage in an active matrix liquid crystal display is disclosed in Technical Bulletin EID95-99, ED95-173, SDM95-213 (1996-02), CORPORATION; ELECTRIC INFORMATION COMMUNICATION SOCIETY.
The feedthrough voltage will be described in brief below. FIG. 11 shows the relation of voltage waveforms of respective parts of a thin film transistor and a pixel electrode when the thin film transistor arranged in the active matrix circuit is operated.
In FIG. 11, Vg is a signal voltage supplied from a gate signal line. Vs is a signal voltage supplied from a drain wiring. Vd is an output voltage of the thin film transistor. Vd is a waveform of a voltage applied to a liquid crystal. First, when the gate voltage Vg rises to an ON level Vgh, the thin film transistor turns ON, and the voltage signal Vs supplied from the source signal line is applied to the liquid crystal through the thin film transistor. FIG. 11 shows the waveform of the voltage Vd applied to the liquid crystal.
After the gate voltage Vg falls to an OFF level Vg1, an electric field is kept applying to the liquid crystal by electric charge charged in the liquid crystal and auxiliary capacitance.
When a next pulse of the gate voltage Vg is inputted to the gate electrode, picture image information to the pixel electrode is rewritten. That is, when the next pulse of the gate voltage Vg is inputted to the gate electrode, the thin film transistor is again turned ON, so that electric charges corresponding to a new signal voltage Vs are flown into the pixel electrode.
Generally, in order to prevent deterioration of the liquid crystal, an alternating voltage expressed by Vsigc.+-.Vsig is used as the voltage Vd. Here, Vsigc is a center voltage, and Vsig is an image signal voltage. The value of Vsig corresponds to a gradation.
In driving such a thin film transistor, when the thin film transistor is switched from an ON state to an OFF state, the trailing voltage (falling voltage) of the gate voltage Vg gives variation to a source voltage through parasitic capacitance between a gate and a source. This varying voltage is the feedthrough voltage (.DELTA.Vs).
FIG. 11 shows the state in which the voltage Vd appearing at the pixel electrode is influenced by the feedthrough voltage (.DELTA.Vs). The feedthrough voltage (.DELTA.Vs) is expressed by the following expression (1). EQU .DELTA.Vs=(1/Ct)[Cgd.multidot..DELTA.Vg-.intg.Idt] (1)
Here, Ct is capacitance of all pixels. The capacitance of all pixels is mainly determined by the addition of capacitance between pixel electrodes and opposite electrodes through a liquid crystal and auxiliary capacitance.
Cgd is parasitic capacitance between a gate and a drain. .DELTA.Vg is a variation amount of gate voltage. In the case shown in FIG. 11, .DELTA.Vg is expressed by .DELTA.Vg=Vgh-Vg1.
.intg.Idt is a term expressing the influence due to a compensation current flowing between a source and a drain, which is caused by the distortion of waveform of the signal voltage supplied from the gate signal line.
As shown in FIG. 10, the signal waveform transmitting through the gate wiring becomes a somewhat distorted form not a completely rectangular wave, as the signal transmits through the gate signal line. Especially, the falling portion of the signal becomes a waveform with a trail.
This is caused from the low characteristics of a gate driver circuit and further from a time constant determined by the product of wiring resistance and wiring capacitance.
FIG. 10 shows the state in which even if the peripheral drive circuit 11 exerts complete driving force to feed a completely rectangular wave 13, a transmitted signal waveform 14 is distorted by a time constant determined by the product of wiring resistance and wiring capacitance.
In the case where the peripheral drive circuit is constituted by the thin film transistors, it is difficult to send out a completely rectangular waveform in the existing circumstance. This is because it is difficult to obtain the thin film transistors having characteristics required for forming the peripheral drive circuit.
In the case where the thin film transistor is driven by the distorted waveform 14 as shown in FIG. 10, it takes a predetermined time until the thin film transistor turns completely OFF. In this period, a current flows in the direction to correct the feedthrough voltage. The term indicated by .intg.Idt of expression (1) expresses the total amount of this current.
In the thin film transistor using an amorphous silicon film, since the mobility is small to be 1cm.sup.2 /Vs or less and an area of active layer is large (of course, also a channel area is large), the capacitance generated by charges flowing through a channel and electric charges stored in the channel makes a large contribution to the parasitic capacitance Cgd.
On the other hand, contribution of a value of I of the term indicated by .intg.Idt is small since in the thin film transistor using the amorphous silicon film, the mobility is small to be 1cm.sup.2 /Vs or less.
Further, since a driver IC is used for the gate driver circuit, the distortion of the gate signal is not so large.
Accordingly, in the case where the thin film transistor using the amorphous silicon film is used, the contribution of the term indicated by .intg.Idt is small.
Accordingly, in the case where the thin film transistor using the amorphous silicon film is used, the dispersion of the feedthrough voltage is mainly caused from the first term of expression (1). Especially, the dispersion of the parasitic capacitance Cgd becomes a main cause.
On the other hand, in a thin film transistor using a crystalline silicon film, the mobility is large, and the area of a gate electrode is small, so that the value of the parasitic capacitance Cgd is small as compared with the case of the thin film transistor using the amorphous silicon film. Also, since a channel area is small, the contribution of capacitance generated by electric charges flowing through the channel and electric charges stored in the channel is not so large.
The thin film transistor using the crystalline silicon film has a large mobility of several tens cm.sup.2 /Vs or more. However, the dispersion of the value is relatively large. In the peripheral circuit integration type, since the peripheral drive circuits 11 and 12 are constituted by thin film transistors, the distortion of gate signal voltage as shown by the waveform 14 in FIG. 10 also becomes large.
The fact that the distortion of the gate signal voltage is large means that the integration range of the second term in expression (1) becomes large. The distortion of the gate signal voltage reflects the influence of dispersion of mobility of thin film transistors constituting the peripheral drive circuit.
Thus, in the active matrix circuit integrated with a peripheral drive circuit constituted by thin film transistors using the crystalline silicon film, the dispersion of the feedthrough voltage expressed by expression (1) is influenced more strongly by the second term than the first term.
That is, by the dispersion of the mobility of the thin film transistors constituting the peripheral drive circuit, dispersion (which relates to distortion of the gate signal waveform 14 as shown in FIG. 10) occurs in the integration range of the second term of expression (1), and further by the dispersion of the mobility of the thin film transistors arranged at every pixel, dispersion occurs at the value of I of the second term of expression (1). These are combined to produce dispersion in the feedthrough voltage.